In recent years, from the viewpoint of approaching to environmental issues such as reduction of carbon dioxide emission, there has arisen an urgent need that automobiles, construction machines and agricultural machines consume less energy, i.e., are reduced in the fuel-consumption thereof. In particular, there is a growing demand that their units such as engines, transmissions, final reduction gear units, compressors and hydraulic equipment contribute to energy saving. Therefore, the lubricating oils used in these units are demanded to be less in frictional loss by agitation and frictional resistance than ever before.
Lowering the viscosity of a lubricating oil may be an example as a means for improving the fuel economy by a transmission and a final reduction gear unit. For example, an automobile automatic transmission or continuously variable transmission has a torque converter, a wet clutch, a gear bearing mechanism, an oil pump and a hydraulic control system while a manual transmission or final reduction gear unit has a gear bearing mechanism. Lowering the viscosity of the lubricating oil to be used in such transmissions can reduce the stirring and frictional resistances of the torque converter, wet clutch, gear bearing mechanism and oil pump and thus enhance the power transmission efficiency thereof, resulting in an improvement in the fuel economy performance of the automobile.
However, lowering the viscosity of the lubricating oil used in these transmissions causes the above-described units and mechanisms thereof to be significantly shortened in fatigue life and may generate seizure resulting in some malfunctions in the transmissions. In particular when a low viscosity lubricating oil is blended with a phosphorus-based extreme pressure additive to enhance the extreme pressure properties, the fatigue life will be extremely shortened. Therefore, it is generally difficult to lower the viscosity of the lubricating oil. It is generally known that although a sulfur-based extreme pressure additive can improve the fatigue life of transmissions, the viscosity of the base oil gives a more effect on the fatigue life than additives under low lubricating conditions.
Examples of conventional automobile transmission oils which can render a transmission capable of maintaining various properties such as shifting properties for a long time include those obtained by optimizing and blending synthetic and/or mineral base oils, antiwear agents, extreme pressure additives, metallic detergents, ashless dispersants, friction modifiers and viscosity index improvers (for example, see Patent Documents 1 to 4 below). However, these compositions are not aimed at improving the fuel economy performance of an automobile and thus are high in kinematic viscosity. Any of the publications does not refer to effects on the fatigue life obtained by lowering the viscosity of the lubricating oils at all. Therefore, a composition which can solve the foregoing problems has not been sufficiently studied yet.                (1) Japanese Patent Laid-Open Publication No. 3-39399        (2) Japanese Patent Laid-Open Publication No. 7-268375        (3) Japanese Patent Laid-Open Publication No. 2000-63869        (4) Japanese Patent Laid-Open Publication No. 2001-262176        